In an image input device using CCDs (charge coupled devices) as an image capture device, a method has been used for obtaining from each photoelectric transducer pseudo signal levels of R, G, and B colors, in which one kind of color filter for any of red (R color), green (G color), and blue (B color) is attached on each photoelectric transducer, an image capture device composed of photoelectric transducers two-dimensionally arranged as pixels for forming images is used, and color signals that cannot be obtained from each photoelectric transducer (interpolation colors) regarding image information obtained by imaging are calculated using color signals of the same color obtained from other photoelectric transducers arranged around that photoelectric transducer. FIG. 10 illustrates an example in which photoelectric transducers are arranged in a general Bayer pattern alignment. In addition, each photoelectric transducer is in charge of each pixel of an image captured by an image capture device. Moreover, the color of a pixel in the position at row m and column n depends on a signal (a capture color signal) obtained from the photoelectric transducer disposed in the position.
As a method of obtaining a color image from image information obtained from the image capture device by performing color signal interpolation, a conventional linear interpolation method will be described (hereinafter referred to as a conventional technology 1). Only one kind of color signal (capture color signal) can be obtained from one photoelectric transducer. Signals of other colors that cannot be obtained from the one photoelectric transducer (non-capture color signals) are generated from output signals from other photoelectric transducers arranged therearound that output signals of the same color as the non-capture color signal. Here, if signals of G color are focused on, G signals obtained by capturing (expressed in capital letters “G”) are present at positions illustrated in FIG. 11. For a pixel of interest, for which a color signal is generated by interpolation, the mean value is calculated by the linear interpolation method from signals of four adjacent pixels on the right, left, top, and bottom, of the pixel of interest, to generate a G signal for a pixel for which no G signal is present (expressed in a small letter “g”), whereby G signals for all the pixels can be obtained. Meanwhile, if B signals are focusing on, as illustrated in FIG. 12, a signal b1 is generated from B signals of two upper and lower adjacent pixels, a signal b2 is generated from B signals of four adjacent pixels on the right, left, top, and bottom, and a signal b3 is generated from B signals of two right and left adjacent pixels, whereby B signals for all the pixels are individually obtained by the linear interpolation method or the like. Regarding R signals, as illustrated in FIG. 13, signals for all the pixels can be interpolated according to the same method as that for B signals. According to the above-described methods, R, G, and B signals for all the pixels can be obtained (a low-pass filter, for example, can be used for the linear interpolation).
However, there has been a problem in that the method in the conventional technology 1 cannot achieve sufficient resolutions, and false colors that are not present in the original subject are generated in edge portions in the image. This results from the fact that a single-color filter is disposed, on each pixel to take images. More specifically, because a single-color filter is disposed on each pixel to take images, spatial frequency characteristics and phases are various, so that the linear interpolation method in the conventional technology 1 cannot sufficiently restore high-frequency components, whereby resolutions corresponding to the number of pixels cannot be achieved for any of R, G, and B colors.
In order to resolve such a problem, the applicant has filed an application of Japanese Patent Laid-Open No. 56446/1993 hereinafter referred to as a conventional technology 2) on a method of performing color interpolation using color correlativity with high resolutions while suppressing false colors sufficiently. Hereinafter, the conventional technology 2 will be described.
In FIG. 10, when a pixel of interest is at a position of G at row m and column n, a method of calculating R and B signal levels is performed by calculating the ratio of signal change between different colors as in Formula 1 and Formula 2 (the ratio of RLPF to GLPF in Formula 1, and the ratio of BLPF to GLPF in Formula 2). Here, RLPF means an LPF output value, which is an output value when an R signal is inputted to a low-pass filter (LPF), GLPF means an LPF output value of a G signal, and BLPF means an LPF output value of a B signal. G(m, n) means a capture color signal, which is an output signal obtained by being actually captured by a photoelectric transducer at the position of the G pixel of interest among the capturing pixels. R(m, n) means a signal value of an interpolation color R calculated for the G position, and B(m, n) means a signal value, of an interpolation color B calculated for the G position. Those R(m, n) and B(m, n) can be calculated using G(m, n), which is an actually-imaged capture color signal, according to the following Formula 1 and Formula 2. In addition, the G(m, n) is also referred to as a reference color because the G(m, n) is a color that is referred to in calculating the R(m, n) and the B(m, n).
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      R          ⁡                      (                          m              ,              n                        )                          =                              G            ⁡                          (                              m                ,                n                            )                                ×                                    R              LPF                                      G              LPF                                                          (                  Formula          ⁢                                          ⁢          1                )                                [                  Expression          ⁢                                          ⁢          2                ]                                                                      B          ⁡                      (                          m              ,              n                        )                          =                              G            ⁡                          (                              m                ,                n                            )                                ×                                    B              LPF                                      G              LPF                                                          (                  Formula          ⁢                                          ⁢          2                )            
By generalizing Formula 1 or Formula 2, given that a capture color of a pixel of interest is expressed as a color J, and the position is expressed as (m, n), a calculation method of generating by interpolation a color H, which is a kind of color different from the color J, as an interpolation color for the position (m, n) is expressed in Formula 3.
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                      H          ⁡                      (                          m              ,              n                        )                          =                              J            ⁡                          (                              m                ,                n                            )                                ×                                    H              LPF                                      J              LPF                                                          (                  Formula          ⁢                                          ⁢          3                )            
The method according to the conventional technology 2 utilizes a general feature of captured images that variations in color signals are less than variations in luminance signals, in other words, there are areas in which color correlativity is high between different colors. FIG. 14 is a schematic diagram when generating by interpolation using Formula 3 an R color at the position of a pixel on which a G color filter is present. The diagrams illustrates a case in which a one dimensional image capture device is considered for simplifying explanation. In the diagram, signals indicated by black circles or white circles express the levels of signals of each color, actually captured by the image capture device. A chart in chain double-dashed line expresses a signal variation in the signal of the G color, which is a reference color, and a chart in solid line expresses a desired signal variation in the R color signal that would be correctly interpolated at positions of pixels on which G-color filters are present. In the diagram, R color signals generated by interpolation using the linear interpolation method in the conventional technology 1 are generated as signal levels indicated by the x marks on the double line. In this case, in areas in which variations in the signal level are slow, appropriate signal level values are generated without problems. However, in edge areas of images in which variations in the signal level are precipitous, color signals to be interpolated cannot be sufficiently recreated, so that signal changes between colors are unbalanced, whereby the color signals are perceived as false colors that are originally not present. In the meanwhile, color signals generated when interpolation colors are interpolated using the method in the conventional technology 2 are recreated as signal levels indicated by star marks in the diagram. Therefore, full-color images obtained by sufficiently recreating edges in the images have high resolutions and high qualities while including less false colors.
However, in the method in the conventional technology 2, because the rate of local change in signal levels of the reference color is used in obtaining interpolation colors, specific deterioration in image quality sometimes arises. FIG. 15 is a schematic diagram illustrating a case in which deterioration in images quality arises by performing color interpolation using the method in the conventional technology 2. As illustrated in the diagram, in a case in which LPF output values of referenced signal levels are small, the sensor output signal varies due to dark noise or the like, and LPF output values of signal levels to be generated by interpolation are high, in other words, in chromatic color areas, the interpolated color signal largely varies in proportion to variation in the reference color signal, so that black spots or white spots that are not present in the original image are sometimes generated in pixels of interest.
For example, assuming an imaging device in which R, G, and B signals are expressed by 10 bits each (in the range from 0 through 1023; 0: dark, 1023: bright), and given that local signal levels are G1=4, G3=1, G5=4, and R2=R4=1023 in FIG. 15, and that each LPF value is calculated by simple averaging, the calculation in Formula 3 is performed specifically by the calculations in Formula 4 through Formula 6.[Expression 4]GLPF=(4+1+4)/3=3  (Formula 4)[Expression 5]RLPF=(1023+1023)/2=1023  (Formula 5)[Expression 6]r3=G3×RLPF/GLPF=1×1023/3=341  (Formula 6)
Therefore, the generated interpolation signal value r3 is an extremely small value compared with signal levels of pixels of the same color arranged therearound. More specifically, although the conventional technology 2 is an effective method with a high image quality and less false colors as a method of interpolating color signals in an imaging device with an image capture device, there has been a problem in that method-specific image quality deterioration is caused depending on effects of noise characteristic of sensors or on imaged pictures.    Patent document 1: Japanese Patent Laid-Open No. 56446/1993    Patent document 2: Japanese Patent Laid-Open No. 078211/2001    Patent document 3: Japanese Patent Laid-Open No. 165894/2000    Non-patent document 1: “National Technical Report” (Vol. 31; No. 1, February 1985), published by Matsushita Techno Research, Inc., released by Ohmsha, Ltd.